1. Technical Field
The invention relates to the field of solar cells, and more particularly to a solar cell which includes an array of long and short wavelength photovoltaic cells on a substrate.
2. Background Art
Solar cells are solid-state devices that directly convert sunlight into electricity. This conversion is typically called the photovoltaic effect, which is the physical process through which the solar cell converts photons from incident sunlight directly into electricity—with the photon energy (Eph) proportional to the incident sunlight's wavelength (λ) with Eph=1.24/λ. Incident light on the solar cell produces both a DC current (I) and voltage (V) that delivers electric power (Power=V×I) to an external load.
Under illumination, incident photons are absorbed, creating electron-hole pairs. The electron-hole pairs are separated and collected by an internal electric field—typically a PN junction—followed by the movement of higher energy electrons from the solar cell into an external circuit. The electrons then dissipate their energy in the external load and return to the solar cell—ultimately recombining with holes. The DC current (I) is proportional to the intensity of the incident sunlight, while the DC voltage (V) is related to the energy band-gap (Eg) of the solar cell material. For example, a solar cell under more intense sunlight, such as under optical concentration, will generate more DC current (FIG. 2), while a solar cell made of a material with a wider band-gap (Eg) will result in a higher DC voltage.
At a slightly more detailed level (FIG. 3), incident photons are absorbed and generate electron-hole pairs if their energy is equal to (or greater) than the energy band-gap (Eg) of the solar cell material (case 2 and 3)—while photons with energy less than the band-gap (case 1) are not absorbed and do not generate electrical current. For case 3, photons with energy (Eph) greater than the band gap, the excess photon energy is lost as heat since the carriers will instantaneously thermalize (decay) in energy to the edges of the band-gap.
Solar cell efficiency equals the generated power (V×I) divided by the incident power. Hence, for a fixed incident power, higher efficiency implies that the cells output voltage and/or current must be increased. For a solar cell material with single band-gap (Eg), sunlight's broad spectrum sets a limit on how much power (V×I) can theoretically be generated. This concept is best illustrated by referring to FIG. 4. First, assume a single-junction solar cell material with a band-gap (Eg) of 2.4 eV. Incident photons with energy (Eph) less than the band-gap energy of 2.4 eV will not be absorbed and, of course, will not generate any current (I). Looking at the solar spectrum plot in FIG. 4, greater than 75% of incident sunlight consists of photons with energies less than 2.4 eV which are not absorbed and thus will not contribute to the output current. So, while the DC voltage for a large 2.4 eV band-gap is more than double that of a 1.1 eV bandgap (silicon) cell, the efficiency of the 2.4 eV cell will be less than 25%, since 75+% of incident sunlight is not collected.
Alternatively, for a solar cell with a narrow band-gap (Eg) of 0.95 eV, all sunlight with a wavelength less than 1.3 μms is absorbed, which from the curve, translates to greater than 75% of the incident sunlight being absorbed. However, while the DC current is maximized with a small Eg (0.95 eV), the output voltage (V) is much less, limiting the maximum solar cell efficiency to approximately 30% (V×I). Again, this is case 3 (FIG. 3), where the voltage difference between the photon energy (1.24/λ) and the energy band-gap (Eg) is directly lost to heat within the solar cell due to the generated carriers (electron-hole pairs) instantaneously decaying (thermalizing) to the band-gap edges.
Single energy band-gap (single junction) solar cells are considered to have a fairly low theoretical upper efficiency limit. To get around this inherent physical limitation, one method to increase theoretical solar cell efficiency is to simply construct a solar cell more than one band-gap (Eg). As illustrated in FIG. 5, the conventional approach is to vertically stack a wide bandgap semiconductor on top of progressively narrower bandgap semiconductors (a.k.a., vertical Tandem Solar Cells). The top cells are able to more efficiency absorb higher energy photons, minimizing thermalization losses and allowing lower energy photons to be transmitted to the progressively narrower band-gap layers. However, tandem cells require extremely complex processing for both lattice and current matching; typically can not be contacted separately; plus require complex and transparent vertical tunnel junctions to connect in a series manner the vertically stacked solar cells. Lattice mismatch can lead to a number of problems that degrade energy efficiency, including high defect density, rough surface morphology—plus epitaxial layer cracking, bowing, and/or warping (i.e., vertically stacked tandem cells can end up resembling both the shape as well as the surface of a potato chip). See, 1. Barnett, et. al., “50% Efficient Solar Cell Architectures and Designs,” IEEE 1-4244-0016-3/06, 4th World Conference on Photovoltaic Energy Conversion, 2006 (referred to as “Burnett”).
An approach that allows the different bandgap solar cells to be manufactured separately and placed adjacent to each other (laterally) and thus contacted separately eliminates the need for precise current and lattice matching along with somewhat reducing the manufacturing complexity associated with vertical tandem solar cells. However, laying single-bandgap cells next to each other (instead of being vertically stacked) eliminates the efficiency gains—unless the incident sunlight can somehow be spectrally split and then concentrated.
FIG. 6 (see Burnett) illustrates a recent lateral solar cell architecture (and a prior art example) where the incident sunlight is split into three spectral components (red/green/blue) and then focused using a static concentrator onto three separately contacted (adjacent) solar cells—each designed with an optimum band-gap for the incident light spectrum. From Burnett, “The lateral solar cell architecture increases the choice of materials for multiple junction solar cells, since it avoids lattice and current matching constraints. Further, since the devices do not need to be series connected, spectral mismatch losses are reduced. Finally, by contacting the individual solar cells with individual voltage busses, the need for tunnel junctions is avoided. Since each material requires unique (and transparent) tunnel junction contact metallurgy, eliminating tunnel junctions represents a substantial simplification”.
The theoretical efficiency for an optimum band-gap single lateral solar cell architecture type device, as shown in FIG. 6 as an example, should show theoretical efficiency improvements for these multi-junction solar cells. For example, the theoretical efficiency for an optimum band-gap single junction solar cell may be 32.4%, while a three-junction solar cell (lateral or tandem) may have a theoretical efficiency of 50.3%. See, Martin A. Green, “Solar Cells, Operating Principles, Technology, and System Applications”, 1992.
In general a multi-junction, monolithic, photovoltaic solar cell device is provided for converting solar radiation to photocurrent and photovoltage with improved efficiency. Such a solar cell device comprises a plurality of semiconductor cells, i.e., active p/n junctions, connected in tandem and deposited on a substrate fabricated from Gallium Arsenide (GaAs) or Germanium (Ge). To increase efficiency, each semiconductor cell is fabricated from a crystalline material with a lattice constant substantially equivalent to the lattice constant of the substrate material. Additionally, the semiconductor cells are selected with appropriate band gaps to efficiently create photovoltage from a larger portion of the solar spectrum. In this regard, one semiconductor cell in each embodiment of the solar cell device has a band gap between that of Ge and GaAs. To achieve desired band gaps and lattice constants, the semiconductor cells may be fabricated from a number of materials including Germanium (Ge), Gallium Indium Phosphide (GaInP), Gallium Arsenide (GaAs), GaInAsP, GaInAsN, GaAsGe, BGaInAs, (GaAs)Ge, CuInSSe, CuAsSSe, and GaInAsNP, for example.
By now it should be clear that a lateral multi-junction solar cell architecture is very appealing for quite a few reasons. However, as evident in FIG. 6 (prior art), the illustrated lateral solar cell architecture is not very compact—as compared to a vertical tandem multi-junction cell—with the ‘add-on’ optics increasing the overall solar cell thickness by at least 1 cm. Additionally, the illustrated ‘bulky’ macro-optics configuration results in quite a bit of empty or unused space between the solar cells—increasing the overall solar panel area as compared to a comparable performance vertical tandem cell. In addition to not being very compact (thin), known lateral solar cells (see FIG. 6 as an example) also involve a fairly complex and expensive assembly process that requires precise alignment of external optics to the individual solar cells.
For terrestrial or even spacecraft-based portable power and recharging applications where compactness combined with high efficiency is key due the low available surface area—such as on a cell phone, Blackberry, iPod, or a Soldiers Helmet—the ‘ideal’ solar cell architecture would be one that offers the ‘compact’ (thin and light) high conversion efficiency of a vertical multi-junction tandem, but with the inherent ‘simplicity’ and potentially lower manufacturing cost of a lateral architecture multi-junction solar cell.
While the above cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.